Structural adhesives are well-known for bonding metal to metal, metal to plastic and plastic to plastic. Structural adhesives are attractive alternatives to mechanical joining methods, such as riveting or spot welding, because structural adhesives distribute load stresses over larger areas rather than concentrating such stresses at a few points. Use of structural adhesives can reduce or eliminate finishing costs because fasteners need not be hidden to produce an aesthetically pleasing appearance. Structural adhesives also produce cleaner and quieter products because they can seal out water, dust and noise. Additionally, they can be used to bond a diversity of materials without extensive surface preparation.
Despite their attractiveness, known structural adhesives have several potential drawbacks. While known structural adhesives have good high temperature performance and good durability, the bond they create is rigid. A rigid bond unevenly distributes stress within the bond, meaning that the stress at the edges of the of bond are higher then the stress in the middle of the bond. Thus, when two workpieces are bonded together at an overlap, increasing the overlap size does not significantly increase the bond strength. Furthermore, the uneven stress of rigid structural adhesives can lead to workpiece damage because adhesive will not fail but will remove paints and coating or in some cases break fiber reinforced plastics.
Additionally, structural adhesives which overcome the rigidity problem are known. These elastic adhesives evenly distribute stress within the bond. The result of this phenomenon is that loads are efficiently absorbed and distributed. However, known flexible structural adhesives have unacceptably poor high temperature performance and poor durability.
Consequently, the inventors have recognized the need for a structural adhesive which has excellent flexibility, good high temperature performance, and good durability, while also being useable without significant surface preparation. The inventors have also recognized the need for a structural adhesive which causes little damage to painted or coated workpieces.